fix(replication): fail SSE-C passthrough closed on targets that drop transport headers

SSE-C ciphertext passthrough replicates via X-Rustfs-Replication-* transport
headers. A MinIO/generic-S3 target silently discards them, storing bare
ciphertext with no decryption material — yet the PUT succeeded, so the object
reported COMPLETED with a silently unreadable replica (backlog#1675 N2).

Fail-closed design:
- SsecPassthroughCapability {Unknown, Supported, Unsupported} cached in
  BucketTargetSys per target ARN with a recording timestamp. Entries reset
  whenever the target is rebuilt, edited, or removed (arn_remotes_map
  lifecycle) and expire after SSEC_PASSTHROUGH_CAPABILITY_TTL (10 minutes):
  an expired verdict in either direction is re-earned through the audit, so
  an Unsupported target recovers automatically after an upgrade (at most one
  wasted PUT+HEAD audit per bad target per TTL window) and a Supported
  verdict cannot outlive a backend swapped behind the same endpoint.
- Replication worker (replicate_object and replicate_all): fresh Unsupported
  targets never receive the PUT — the attempt fails immediately into the
  normal MRF retry channel with a "run ?replication-check to re-probe" hint.
  Unknown or expired verdicts are audited: after the PUT the worker HEADs
  the replica back through the replication-check channel (source version id
  mapped through resolve_read_api_version_id, so null-version objects audit
  correctly) and requires SSE-C evidence (the echoed customer-algorithm
  header); missing evidence records Unsupported and fails the attempt.
  Convergence HEADs are audited the same way, so a broken ciphertext replica
  from an earlier attempt can never launder itself into COMPLETED via an
  ETag match. The gate/evidence policy is pure (replication_target_boundary,
  staleness folded in as an input) for the M2 worker migration.
- replication-check grows an SsecPassthrough probe phase: a probe PUT
  carrying the live transport-header shape, HEAD-back for evidence, and a
  machine-readable Code BucketRemoteSsecPassthroughUnsupported on failure.
  The probe verdict is synced into the runtime capability cache. Unlike
  VersionFidelity, a failed SsecPassthrough phase does NOT fail the target
  overall — it is a capability limit, not a broken replication contract,
  and a plaintext-only deployment against such a target must not turn red.
- fake_s3_target: default mode now models a RustFS target (stores the
  transport headers, echoes SSE-C evidence); the new
  drop_unlisted_replication_headers mode models MinIO. The journal records
  whether a request carried transport headers.

Receiver-echo verification: the replication-check HEAD exemption only skips
SSE-C key validation; the response has always built sse-customer-algorithm
from stored metadata (rustfs/src/app/object_usecase.rs), so no receiver
change was needed — pinned end to end by the replication-check e2e against
a real RustFS target.

Rolling-upgrade constraint: RustFS targets older than the replication-check
HEAD exemption (#5898) answer the audit HEAD without SSE-C evidence (or fail
it outright), so SSE-C replication to such targets reports FAILED. This is
deliberate — FAILED-and-retryable beats a silently undecryptable replica —
and self-heals: once the target is upgraded, the next TTL expiry (or a
manual ?replication-check re-probe) re-audits and records Supported.
Plaintext and managed-SSE replication are unaffected. The capability cache
is per-node; each node audits independently.

Known limitations:
- The audit judges evidence from the echoed customer-algorithm header only.
  A hypothetical target that preserves that one header while dropping other
  transport headers (partial-drop) would pass the audit; no known target
  behaves this way — observed targets drop the whole unknown-header family.
- A mixed-version target cluster can flap the verdict between audits routed
  to different target nodes until the rollout completes; the TTL bounds how
  long each stale verdict persists.

New e2e (backlog#1675 C1 + N2, red-first): fail-closed against a
header-dropping fake (FAILED + no second PUT via the capability cache,
journal-asserted; red run showed the old COMPLETED), replication-check
reports the SsecPassthrough phase Code while the target stays OK overall,
SSE-C heal convergence after a real target outage, and SSE-C
existing-object resync landing a REPLICA readable with the customer key.
TTL expiry in both directions is pinned at the cache and gate seams.

crates/e2e_test/src/fake_s3_target/mod.rs

@@ -90,6 +90,13 @@ const SOURCE_LEGALHOLD_TIMESTAMP_HEADERS: [&str; 2] = [
     "x-rustfs-source-replication-legalhold-timestamp",
     "x-minio-source-replication-legalhold-timestamp",
 ];
+/// Wire prefix of the SSE-C passthrough replication transport headers
+/// (`X-Rustfs-Replication-*`). In the default mode the fake stores them like a
+/// RustFS target and echoes SSE-C evidence back on HEAD/GET; with
+/// [`FakeS3Target::drop_unlisted_replication_headers`] it models MinIO /
+/// generic S3, which silently discard unknown x-* headers.
+const REPLICATION_SSE_TRANSPORT_PREFIX: &str = "x-rustfs-replication-";
+const REPLICATION_SSEC_ALGORITHM_TRANSPORT_HEADER: &str = "x-rustfs-replication-ssec-algorithm";
 const RESERVED_BUCKET_PREFIXES: [&str; 3] = ["xn--", "sthree-", "amzn-s3-demo-"];
 const RESERVED_BUCKET_SUFFIXES: [&str; 6] = ["-s3alias", "--ol-s3", ".mrap", "--x-s3", "--table-s3", "-an"];
 
@@ -166,6 +173,10 @@ pub struct ProxyHeaderSnapshot {
     pub ssec_algorithm: Option<String>,
     pub ssec_key_present: bool,
     pub ssec_key_md5: Option<String>,
+    /// Whether the request carried any `X-Rustfs-Replication-*` SSE-C
+    /// passthrough transport header, so fail-closed tests can assert the
+    /// sender really shipped the material a dropping target discarded.
+    pub ssec_transport_present: bool,
 }
 
 impl ProxyHeaderSnapshot {
@@ -179,6 +190,9 @@ impl ProxyHeaderSnapshot {
                 .map(bounded_journal_value),
             ssec_key_present: headers.contains_key("x-amz-server-side-encryption-customer-key"),
             ssec_key_md5: header_value(headers, &["x-amz-server-side-encryption-customer-key-md5"]).map(bounded_journal_value),
+            ssec_transport_present: headers
+                .keys()
+                .any(|name| name.as_str().starts_with(REPLICATION_SSE_TRANSPORT_PREFIX)),
         }
     }
 }
@@ -213,6 +227,10 @@ struct ControlState {
 struct StoreState {
     assign_own_version_ids: bool,
     assign_own_multipart_version_ids: bool,
+    /// MinIO-like mode: silently discard non-whitelisted replication
+    /// transport headers instead of storing them (see
+    /// [`REPLICATION_SSE_TRANSPORT_PREFIX`]).
+    drop_unlisted_replication_headers: bool,
     buckets: HashMap<String, BucketState>,
     uploads: HashMap<String, MultipartState>,
     total_bytes: usize,
@@ -237,6 +255,9 @@ struct ObjectVersion {
     /// Object tags as ordered key/value pairs (PutObjectTagging replaces the
     /// whole set, DeleteObjectTagging clears it).
     tags: Vec<(String, String)>,
+    /// SSE-C passthrough transport headers stored with the version (RustFS
+    /// target behavior); empty when the drop mode discarded them.
+    replication_sse_headers: Vec<(String, String)>,
 }
 
 #[derive(Clone)]
@@ -246,6 +267,7 @@ struct MultipartState {
     version_id: String,
     content_type: Option<String>,
     metadata: Option<HashMap<String, String>>,
+    replication_sse_headers: Vec<(String, String)>,
     parts: BTreeMap<i32, MultipartPart>,
 }
 
@@ -466,6 +488,15 @@ impl FakeS3Target {
 
     /// Mint own version ids for the multipart path only — models a target
     /// that adopts PutObject version ids but not CreateMultipartUpload ones.
+    /// MinIO-like mode: silently drop every `X-Rustfs-Replication-*` SSE-C
+    /// passthrough transport header instead of storing it. The default (off)
+    /// models a RustFS target, which preserves the headers and echoes SSE-C
+    /// evidence (`x-amz-server-side-encryption-customer-algorithm`) on
+    /// HEAD/GET of the replica.
+    pub fn drop_unlisted_replication_headers(&self, enabled: bool) {
+        lock(&self.backend.store).drop_unlisted_replication_headers = enabled;
+    }
+
     pub fn assign_own_multipart_version_ids(&self, enabled: bool) {
         lock(&self.backend.store).assign_own_multipart_version_ids = enabled;
     }
@@ -842,6 +873,29 @@ fn new_version_id(headers: &HeaderMap, assign_own: bool) -> S3Result<String> {
     Ok(version_id.to_string())
 }
 
+/// Capture the SSE-C passthrough transport headers a replication PUT carried.
+/// Returns an empty set in the MinIO-like drop mode.
+fn captured_replication_sse_headers(headers: &HeaderMap, drop_unlisted: bool) -> Vec<(String, String)> {
+    if drop_unlisted {
+        return Vec::new();
+    }
+    headers
+        .iter()
+        .filter(|(name, _)| name.as_str().starts_with(REPLICATION_SSE_TRANSPORT_PREFIX))
+        .filter_map(|(name, value)| Some((name.as_str().to_string(), value.to_str().ok()?.to_string())))
+        .collect()
+}
+
+/// SSE-C evidence a RustFS-like target echoes for a stored passthrough
+/// replica: the customer algorithm restored from the transport headers.
+fn stored_sse_customer_algorithm(version: &ObjectVersion) -> Option<String> {
+    version
+        .replication_sse_headers
+        .iter()
+        .find(|(name, _)| name == REPLICATION_SSEC_ALGORITHM_TRANSPORT_HEADER)
+        .map(|(_, value)| value.clone())
+}
+
 fn source_etag(headers: &HeaderMap) -> S3Result<Option<String>> {
     header_value(headers, &SOURCE_ETAG_HEADERS)
         .map(|value| validate_retained_identifier(value, "source ETag").map(|value| normalize_etag(&value)))
@@ -1312,7 +1366,10 @@ impl S3 for FakeBackend {
         let input = req.input;
         let body = collect_stream(input.body, input.content_length, fault.as_ref(), &self.control).await?;
         validate_stored_metadata(&input.content_type, &input.metadata)?;
-        let assign_own = lock(&self.store).assign_own_version_ids;
+        let (assign_own, drop_unlisted) = {
+            let state = lock(&self.store);
+            (state.assign_own_version_ids, state.drop_unlisted_replication_headers)
+        };
         let version_id = new_version_id(&headers, assign_own)?;
         let e_tag = match source_etag(&headers)? {
             Some(value) => value,
@@ -1330,6 +1387,7 @@ impl S3 for FakeBackend {
             content_type: input.content_type,
             metadata: input.metadata,
             tags: Vec::new(),
+            replication_sse_headers: captured_replication_sse_headers(&headers, drop_unlisted),
         };
         upsert_version(&mut lock(&self.store), &input.bucket, input.key, version)?;
         Ok(apply_response_fault(
@@ -1350,6 +1408,7 @@ impl S3 for FakeBackend {
             let state = lock(&self.store);
             find_version(&state, &input.bucket, &input.key, input.version_id.as_deref())?
         };
+        let sse_customer_algorithm = stored_sse_customer_algorithm(&version);
         Ok(apply_response_fault(
             S3Response::new(GetObjectOutput {
                 body: Some(StreamingBlob::new(Body::from(version.body.clone()))),
@@ -1359,6 +1418,7 @@ impl S3 for FakeBackend {
                 e_tag: Some(ETag::Strong(version.e_tag)),
                 last_modified: Some(version.last_modified.clone()),
                 version_id: Some(version.version_id),
+                sse_customer_algorithm,
                 ..Default::default()
             }),
             fault.as_ref(),
@@ -1373,6 +1433,7 @@ impl S3 for FakeBackend {
             let state = lock(&self.store);
             find_version(&state, &input.bucket, &input.key, input.version_id.as_deref())?
         };
+        let sse_customer_algorithm = stored_sse_customer_algorithm(&version);
         Ok(apply_response_fault(
             S3Response::new(HeadObjectOutput {
                 content_length: Some(version.body.len() as i64),
@@ -1381,6 +1442,7 @@ impl S3 for FakeBackend {
                 e_tag: Some(ETag::Strong(version.e_tag)),
                 last_modified: Some(version.last_modified.clone()),
                 version_id: Some(version.version_id),
+                sse_customer_algorithm,
                 ..Default::default()
             }),
             fault.as_ref(),
@@ -1530,6 +1592,7 @@ impl S3 for FakeBackend {
                 content_type: None,
                 metadata: None,
                 tags: Vec::new(),
+                replication_sse_headers: Vec::new(),
             },
         )?;
         Ok(apply_response_fault(
@@ -1557,9 +1620,10 @@ impl S3 for FakeBackend {
         ensure_upload_budget(&state)?;
         validate_stored_metadata(&input.content_type, &input.metadata)?;
         let upload_id = Uuid::new_v4().to_string();
-        // Read the flag before the mutable borrow of `state.uploads` below
+        // Read the flags before the mutable borrow of `state.uploads` below
         // (and never re-lock the store: the mutex is not reentrant).
         let mint_own = state.assign_own_version_ids || state.assign_own_multipart_version_ids;
+        let drop_unlisted = state.drop_unlisted_replication_headers;
         let version_id = new_version_id(&headers, mint_own)?;
         state.uploads.insert(
             upload_id.clone(),
@@ -1569,6 +1633,7 @@ impl S3 for FakeBackend {
                 version_id,
                 content_type: input.content_type,
                 metadata: input.metadata,
+                replication_sse_headers: captured_replication_sse_headers(&headers, drop_unlisted),
                 parts: BTreeMap::new(),
             },
         );
@@ -1706,6 +1771,7 @@ impl S3 for FakeBackend {
                     version_id: upload.version_id.clone(),
                     content_type: upload.content_type.clone(),
                     metadata: upload.metadata.clone(),
+                    replication_sse_headers: upload.replication_sse_headers.clone(),
                     parts: BTreeMap::new(),
                 },
                 selected,
@@ -1733,6 +1799,7 @@ impl S3 for FakeBackend {
             content_type: upload.content_type,
             metadata: upload.metadata,
             tags: Vec::new(),
+            replication_sse_headers: upload.replication_sse_headers,
         };
         let mut state = lock(&self.store);
         let current = state
@@ -1937,6 +2004,65 @@ mod tests {
         Ok(())
     }
 
+    /// Default mode is RustFS-like: SSE-C passthrough transport headers are
+    /// stored and the customer algorithm is echoed on HEAD/GET. Drop mode is
+    /// MinIO-like: the headers are silently discarded, so no evidence comes
+    /// back — the exact difference the N2 fail-closed audit keys on. Both
+    /// modes journal that the sender shipped the transport headers.
+    #[tokio::test]
+    async fn ssec_passthrough_headers_echo_and_drop_modes() -> Result<(), BoxError> {
+        let target = FakeS3Target::start().await?;
+        target.create_bucket("target-bucket");
+        let client = client(&target);
+
+        let put_with_transport_headers = |key: &'static str| {
+            client
+                .put_object()
+                .bucket("target-bucket")
+                .key(key)
+                .body(ByteStream::from_static(b"ciphertext"))
+                .customize()
+                .map_request(move |mut request| {
+                    let headers = request.headers_mut();
+                    headers.insert("x-rustfs-replication-ssec-algorithm", "AES256");
+                    headers.insert("x-rustfs-replication-ssec-key-md5", "AAAAAAAAAAAAAAAAAAAAAA==");
+                    Ok::<_, std::convert::Infallible>(request)
+                })
+                .send()
+        };
+
+        put_with_transport_headers("kept").await?;
+        let head = client.head_object().bucket("target-bucket").key("kept").send().await?;
+        assert_eq!(head.sse_customer_algorithm(), Some("AES256"));
+        let get = client.get_object().bucket("target-bucket").key("kept").send().await?;
+        assert_eq!(get.sse_customer_algorithm(), Some("AES256"));
+
+        target.drop_unlisted_replication_headers(true);
+        put_with_transport_headers("dropped").await?;
+        let head = client.head_object().bucket("target-bucket").key("dropped").send().await?;
+        assert_eq!(head.sse_customer_algorithm(), None, "drop mode must discard SSE-C evidence");
+
+        let requests = target.requests();
+        for key in ["kept", "dropped"] {
+            let record = requests
+                .iter()
+                .find(|record| record.operation == Operation::PutObject && record.key.as_deref() == Some(key))
+                .expect("PUT must be journaled");
+            assert!(
+                record.proxy_headers.ssec_transport_present,
+                "the journal must prove the sender shipped the transport headers for {key}"
+            );
+        }
+        let plain_head = requests
+            .iter()
+            .find(|record| record.operation == Operation::HeadObject)
+            .expect("HEAD must be journaled");
+        assert!(!plain_head.proxy_headers.ssec_transport_present);
+
+        target.shutdown().await;
+        Ok(())
+    }
+
     macro_rules! assert_sdk_error {
         ($error:expr, $status:expr, $code:expr) => {{
             let error = &$error;
@@ -3202,6 +3328,7 @@ mod tests {
                         version_id: index.to_string(),
                         content_type: None,
                         metadata: None,
+                        replication_sse_headers: Vec::new(),
                         parts: BTreeMap::new(),
                     },
                 );

crates/e2e_test/src/replication_extension_test.rs

@@ -2610,17 +2610,20 @@ async fn test_replication_check_succeeds_with_remote_target() -> Result<(), Box<
 
     assert_eq!(response.status(), StatusCode::OK);
     let payload: serde_json::Value = response.json().await?;
-    assert_eq!(payload["Status"], "OK");
+    assert_eq!(payload["Status"], "OK", "{payload}");
     assert_eq!(payload["ActiveMutation"], true);
     assert_eq!(payload["Targets"].as_array().map(Vec::len), Some(1));
-    assert_eq!(payload["Targets"][0]["Status"], "OK");
-    assert_eq!(payload["Targets"][0]["Phases"]["Put"]["Status"], "OK");
+    assert_eq!(payload["Targets"][0]["Status"], "OK", "{payload}");
+    assert_eq!(payload["Targets"][0]["Phases"]["Put"]["Status"], "OK", "{payload}");
     // A RustFS target adopts the source version id, so the P1-19
     // version-identity probe passes.
-    assert_eq!(payload["Targets"][0]["Phases"]["VersionFidelity"]["Status"], "OK");
-    assert_eq!(payload["Targets"][0]["Phases"]["DeleteMarker"]["Status"], "OK");
-    assert_eq!(payload["Targets"][0]["Phases"]["VersionDelete"]["Status"], "OK");
-    assert_eq!(payload["Targets"][0]["Phases"]["Cleanup"]["Status"], "OK");
+    assert_eq!(payload["Targets"][0]["Phases"]["VersionFidelity"]["Status"], "OK", "{payload}");
+    // A RustFS target preserves the SSE-C passthrough transport headers and
+    // echoes the customer algorithm on the replication-check HEAD (N2).
+    assert_eq!(payload["Targets"][0]["Phases"]["SsecPassthrough"]["Status"], "OK", "{payload}");
+    assert_eq!(payload["Targets"][0]["Phases"]["DeleteMarker"]["Status"], "OK", "{payload}");
+    assert_eq!(payload["Targets"][0]["Phases"]["VersionDelete"]["Status"], "OK", "{payload}");
+    assert_eq!(payload["Targets"][0]["Phases"]["Cleanup"]["Status"], "OK", "{payload}");
 
     let target_client = target_env.create_s3_client();
     let versions = target_client
@@ -4649,6 +4652,410 @@ async fn test_bucket_replication_sse_c_multipart_passthrough() -> TestResult {
     Ok(())
 }
 
+/// N2 (backlog#1675 P1-22): SSE-C passthrough replication to a target that
+/// silently drops the `X-Rustfs-Replication-*` transport headers (MinIO-like
+/// behavior, modeled by the fake target's drop mode) used to report COMPLETED
+/// while the replica had irrecoverably lost its decryption material — the red
+/// light this test was born failing on. Fail-closed contract now under test:
+/// the first attempt PUTs, HEAD-backs the replica, finds no SSE-C evidence,
+/// records the target Unsupported and reports FAILED; a second SSE-C object
+/// fails without any PUT reaching the target (capability cache, proven from
+/// the target journal); plaintext objects still replicate COMPLETED.
+#[tokio::test]
+#[serial]
+async fn test_ssec_replication_fails_closed_when_target_drops_passthrough_headers() -> TestResult {
+    init_logging();
+
+    let target = FakeS3Target::start().await?;
+    let target_bucket = "ssec-drop-dst";
+    target.create_bucket(target_bucket);
+    target.drop_unlisted_replication_headers(true);
+
+    let mut source_env = RustFSTestEnvironment::new().await?;
+    let mut env_vars = replication_fast_env();
+    env_vars.extend_from_slice(LOOPBACK_REPLICATION_TARGET_ENV);
+    env_vars.extend_from_slice(&[("NO_PROXY", "127.0.0.1,localhost"), ("HTTP_PROXY", ""), ("HTTPS_PROXY", "")]);
+    source_env.start_rustfs_server_with_env(vec![], &env_vars).await?;
+
+    let source_bucket = "ssec-drop-src";
+    let source_client = source_env.create_s3_client();
+    source_client.create_bucket().bucket(source_bucket).send().await?;
+    enable_bucket_versioning(&source_env, source_bucket).await?;
+    let target_arn = set_replication_target_with_options(
+        &source_env,
+        source_bucket,
+        ReplicationTargetOptions {
+            endpoint: &target.address(),
+            access_key: FAKE_ACCESS_KEY,
+            secret_key: FAKE_SECRET_KEY,
+            target_bucket,
+            secure: false,
+            skip_tls_verify: false,
+            ca_cert_pem: None,
+        },
+    )
+    .await?;
+    put_bucket_replication(&source_env, source_bucket, &target_arn).await?;
+
+    let customer_key = BASE64_STANDARD.encode(REPL17_SSEC_KEY);
+    let customer_key_md5 = sse_customer_key_md5_base64(REPL17_SSEC_KEY);
+    let put_ssec = |key: &'static str| {
+        source_client
+            .put_object()
+            .bucket(source_bucket)
+            .key(key)
+            .body(ByteStream::from_static(b"ssec fail-closed payload"))
+            .sse_customer_algorithm("AES256")
+            .sse_customer_key(&customer_key)
+            .sse_customer_key_md5(&customer_key_md5)
+            .send()
+    };
+
+    // First SSE-C object: the audit must catch the dropped material.
+    put_ssec("ssec-first.txt").await?;
+    wait_for_source_replication_status(&source_client, source_bucket, "ssec-first.txt", "FAILED", true).await?;
+
+    let requests = target.take_requests();
+    let first_put = requests
+        .iter()
+        .find(|record| record.operation == FakeTargetOperation::PutObject && record.key.as_deref() == Some("ssec-first.txt"))
+        .ok_or("the first SSE-C object must have been PUT (capability was Unknown)")?;
+    assert!(
+        first_put.proxy_headers.ssec_transport_present,
+        "the replication PUT must have shipped the SSE-C transport headers the target then dropped"
+    );
+    assert!(
+        requests.iter().any(|record| {
+            record.operation == FakeTargetOperation::HeadObject
+                && record.key.as_deref() == Some("ssec-first.txt")
+                && record.sequence > first_put.sequence
+                && record.proxy_headers.replication_check.as_deref() == Some("true")
+        }),
+        "the post-PUT HEAD-back audit must have run through the replication-check channel; journal: {requests:?}"
+    );
+
+    // Second SSE-C object: the cached Unsupported verdict fails it closed
+    // before any PUT — including MRF retries of the first object.
+    put_ssec("ssec-second.txt").await?;
+    wait_for_source_replication_status(&source_client, source_bucket, "ssec-second.txt", "FAILED", true).await?;
+    assert!(
+        !target.requests().iter().any(|record| {
+            record.operation == FakeTargetOperation::PutObject
+                && record.key.as_deref() != Some("plain-control.txt")
+                && record.proxy_headers.ssec_transport_present
+        }),
+        "no further SSE-C ciphertext may reach a target recorded Unsupported; journal: {:?}",
+        target.requests()
+    );
+
+    // The gate is scoped to SSE-C: plaintext replication keeps working.
+    source_client
+        .put_object()
+        .bucket(source_bucket)
+        .key("plain-control.txt")
+        .body(ByteStream::from_static(b"plaintext control payload"))
+        .send()
+        .await?;
+    wait_for_source_replication_status(&source_client, source_bucket, "plain-control.txt", "COMPLETED", false).await?;
+    assert!(target.has_object(target_bucket, "plain-control.txt"));
+
+    target.shutdown().await;
+    Ok(())
+}
+
+/// N2 (backlog#1675 P1-22): the admin replication-check must expose the same
+/// verdict operators would otherwise only learn from failing SSE-C objects —
+/// an SsecPassthrough probe phase that fails with the machine-readable
+/// `BucketRemoteSsecPassthroughUnsupported` code against a header-dropping
+/// target, with no probe residue left behind. The target's overall status
+/// stays OK: unlike version-identity drift, dropped passthrough headers are
+/// a capability limit, and a plaintext-only deployment against a MinIO-like
+/// target must not turn red.
+#[tokio::test]
+#[serial]
+async fn test_replication_check_flags_ssec_passthrough_dropping_target() -> TestResult {
+    init_logging();
+
+    let target = FakeS3Target::start().await?;
+    let target_bucket = "ssec-check-dst";
+    target.create_bucket(target_bucket);
+    target.drop_unlisted_replication_headers(true);
+
+    let mut source_env = RustFSTestEnvironment::new().await?;
+    let mut env_vars = replication_fast_env();
+    env_vars.extend_from_slice(LOOPBACK_REPLICATION_TARGET_ENV);
+    env_vars.extend_from_slice(&[("NO_PROXY", "127.0.0.1,localhost"), ("HTTP_PROXY", ""), ("HTTPS_PROXY", "")]);
+    source_env.start_rustfs_server_with_env(vec![], &env_vars).await?;
+
+    let source_bucket = "ssec-check-src";
+    let source_client = source_env.create_s3_client();
+    source_client.create_bucket().bucket(source_bucket).send().await?;
+    enable_bucket_versioning(&source_env, source_bucket).await?;
+    let target_arn = set_replication_target_with_options(
+        &source_env,
+        source_bucket,
+        ReplicationTargetOptions {
+            endpoint: &target.address(),
+            access_key: FAKE_ACCESS_KEY,
+            secret_key: FAKE_SECRET_KEY,
+            target_bucket,
+            secure: false,
+            skip_tls_verify: false,
+            ca_cert_pem: None,
+        },
+    )
+    .await?;
+    put_bucket_replication(&source_env, source_bucket, &target_arn).await?;
+
+    let response = run_replication_check(&source_env, source_bucket).await?;
+    assert_eq!(response.status(), StatusCode::OK);
+    let payload: serde_json::Value = response.json().await?;
+
+    assert_eq!(
+        payload["Status"], "OK",
+        "a capability-only SSE-C failure must not fail the check overall: {payload}"
+    );
+    let target_report = &payload["Targets"][0];
+    assert_eq!(target_report["Status"], "OK", "{payload}");
+    let ssec = &target_report["Phases"]["SsecPassthrough"];
+    assert_eq!(ssec["Status"], "FAILED", "SsecPassthrough phase must fail: {payload}");
+    assert_eq!(
+        ssec["Code"], "BucketRemoteSsecPassthroughUnsupported",
+        "the failure must carry the machine-readable code: {payload}"
+    );
+    // Basic replication of plaintext objects works on this target: every other
+    // phase passes, so the code is the discriminator operators branch on.
+    assert_eq!(target_report["Phases"]["Put"]["Status"], "OK", "{payload}");
+    assert_eq!(target_report["Phases"]["VersionFidelity"]["Status"], "OK", "{payload}");
+    assert_eq!(target_report["Phases"]["DeleteMarker"]["Status"], "OK", "{payload}");
+    assert_eq!(target_report["Phases"]["VersionDelete"]["Status"], "OK", "{payload}");
+    assert_eq!(target_report["Phases"]["Cleanup"]["Status"], "OK", "{payload}");
+
+    // The SSE-C probe PUT must have shipped the real transport header names —
+    // a mangled or missing header set would fail the phase for the wrong
+    // reason and mask a working target.
+    let requests = target.requests();
+    assert!(
+        requests
+            .iter()
+            .any(|record| record.operation == FakeTargetOperation::PutObject && record.proxy_headers.ssec_transport_present),
+        "the SSE-C probe PUT must carry the X-Rustfs-Replication-* transport headers; journal: {requests:?}"
+    );
+
+    // No probe residue, including the SSE-C probe version.
+    let probe_put = requests
+        .into_iter()
+        .find(|record| record.operation == FakeTargetOperation::PutObject)
+        .ok_or("the probe PUT never reached the fake target")?;
+    let probe_key = probe_put.key.ok_or("probe PUT journal record has no key")?;
+    assert!(
+        target.stored_versions(target_bucket, &probe_key).is_empty(),
+        "all probe versions must be cleaned up"
+    );
+
+    target.shutdown().await;
+    Ok(())
+}
+
+/// C1 (backlog#1675 P1-22): heal-path convergence for SSE-C. An SSE-C object
+/// whose live replication failed during a target outage must converge through
+/// the scanner/heal compensation once the target returns — passing the N2
+/// HEAD-back audit against the recovered RustFS target — and the replica must
+/// be readable with the customer key.
+#[tokio::test]
+#[serial]
+async fn test_bucket_replication_sse_c_heals_after_target_outage() -> TestResult {
+    init_logging();
+
+    let (source_env, mut target_env, source_bucket, target_bucket) =
+        build_sse_replication_pair("ssec-heal", false, false).await?;
+    let source_client = source_env.create_s3_client();
+    let key = "ssec-heal-contract.txt";
+    let body = b"repl-22 ssec heal payload".to_vec();
+    let customer_key = BASE64_STANDARD.encode(REPL17_SSEC_KEY);
+    let customer_key_md5 = sse_customer_key_md5_base64(REPL17_SSEC_KEY);
+
+    // Target outage: the SSE-C write cannot replicate.
+    target_env.stop_server();
+
+    source_client
+        .put_object()
+        .bucket(&source_bucket)
+        .key(key)
+        .body(ByteStream::from(body.clone()))
+        .sse_customer_algorithm("AES256")
+        .sse_customer_key(&customer_key)
+        .sse_customer_key_md5(&customer_key_md5)
+        .send()
+        .await?;
+
+    // The failure is observable on the source (SSE-C HEAD needs the key).
+    let deadline = tokio::time::Instant::now() + Duration::from_secs(30);
+    loop {
+        let head = source_client
+            .head_object()
+            .bucket(&source_bucket)
+            .key(key)
+            .sse_customer_algorithm("AES256")
+            .sse_customer_key(&customer_key)
+            .sse_customer_key_md5(&customer_key_md5)
+            .send()
+            .await?;
+        match head.replication_status().map(|status| status.as_str()) {
+            Some("PENDING") | Some("FAILED") => break,
+            other => {
+                if tokio::time::Instant::now() >= deadline {
+                    return Err(format!("source SSE-C object never reported PENDING/FAILED; last status={other:?}").into());
+                }
+                sleep(Duration::from_millis(200)).await;
+            }
+        }
+    }
+
+    // Recover the target in place; the source scanner re-drives the failure.
+    target_env
+        .restart_server_preserving_data(vec![], &[("NO_PROXY", "127.0.0.1,localhost"), ("HTTP_PROXY", ""), ("HTTPS_PROXY", "")])
+        .await?;
+
+    wait_for_source_replication_status(&source_client, &source_bucket, key, "COMPLETED", true).await?;
+
+    // The healed replica is a REPLICA (status surfaces on HEAD) readable with
+    // the customer key.
+    let target_client = target_env.create_s3_client();
+    let replica_head = target_client
+        .head_object()
+        .bucket(&target_bucket)
+        .key(key)
+        .sse_customer_algorithm("AES256")
+        .sse_customer_key(&customer_key)
+        .sse_customer_key_md5(&customer_key_md5)
+        .send()
+        .await?;
+    assert_eq!(
+        replica_head.replication_status().map(|status| status.as_str()),
+        Some("REPLICA"),
+        "the healed copy must carry REPLICA status"
+    );
+    let replica = target_client
+        .get_object()
+        .bucket(&target_bucket)
+        .key(key)
+        .sse_customer_algorithm("AES256")
+        .sse_customer_key(&customer_key)
+        .sse_customer_key_md5(&customer_key_md5)
+        .send()
+        .await?;
+    assert_eq!(replica.sse_customer_algorithm(), Some("AES256"));
+    assert_eq!(replica.body.collect().await?.into_bytes().as_ref(), body.as_slice());
+
+    Ok(())
+}
+
+/// C1 (backlog#1675 P1-22): existing-object resync for SSE-C. An SSE-C object
+/// written BEFORE any replication config must reach the RustFS target through
+/// the existing-object resync (`replicate_all` transport, N2-audited), land as
+/// a REPLICA, and read back with the customer key.
+#[tokio::test]
+#[serial]
+async fn test_bucket_replication_sse_c_existing_object_resync() -> TestResult {
+    init_logging();
+
+    let mut source_env = RustFSTestEnvironment::new().await?;
+    let mut source_process_env = replication_fast_env();
+    source_process_env.extend_from_slice(LOOPBACK_REPLICATION_TARGET_ENV);
+    source_process_env.extend_from_slice(FAST_SCANNER_ENV);
+    source_process_env.extend_from_slice(&[("NO_PROXY", "127.0.0.1,localhost"), ("HTTP_PROXY", ""), ("HTTPS_PROXY", "")]);
+    source_env.start_rustfs_server_with_env(vec![], &source_process_env).await?;
+
+    let mut target_env = RustFSTestEnvironment::new().await?;
+    target_env
+        .start_rustfs_server_without_cleanup_with_env(&[
+            ("NO_PROXY", "127.0.0.1,localhost"),
+            ("HTTP_PROXY", ""),
+            ("HTTPS_PROXY", ""),
+        ])
+        .await?;
+
+    let source_bucket = "ssec-existing-src";
+    let target_bucket = "ssec-existing-dst";
+    let source_client = source_env.create_s3_client();
+    let target_client = target_env.create_s3_client();
+    source_client.create_bucket().bucket(source_bucket).send().await?;
+    target_client.create_bucket().bucket(target_bucket).send().await?;
+    enable_bucket_versioning(&source_env, source_bucket).await?;
+    enable_bucket_versioning(&target_env, target_bucket).await?;
+
+    // The SSE-C object exists before any replication wiring.
+    let key = "ssec-existing-contract.txt";
+    let body = b"repl-22 ssec existing-object payload".to_vec();
+    let customer_key = BASE64_STANDARD.encode(REPL17_SSEC_KEY);
+    let customer_key_md5 = sse_customer_key_md5_base64(REPL17_SSEC_KEY);
+    source_client
+        .put_object()
+        .bucket(source_bucket)
+        .key(key)
+        .body(ByteStream::from(body.clone()))
+        .sse_customer_algorithm("AES256")
+        .sse_customer_key(&customer_key)
+        .sse_customer_key_md5(&customer_key_md5)
+        .send()
+        .await?;
+
+    // Wire replication (existing-object enabled) and drive a resync.
+    let target_arn = set_replication_target(&source_env, source_bucket, &target_env, target_bucket).await?;
+    put_bucket_replication(&source_env, source_bucket, &target_arn).await?;
+    let (reset_arn, reset_id) = start_bucket_replication_reset(&source_env, source_bucket).await?;
+    assert_eq!(reset_arn, target_arn);
+    let terminal = wait_for_replication_reset_target(&source_env, source_bucket, &target_arn, |status| {
+        status.reset_id == reset_id && matches!(status.status.as_str(), "Completed" | "Failed")
+    })
+    .await?;
+    assert_eq!(terminal.status, "Completed", "SSE-C existing-object resync must complete");
+    assert!(terminal.replicated_count >= 1, "the existing SSE-C object must have been resynced");
+
+    // The replica is a REPLICA (status surfaces on HEAD) readable with the
+    // customer key.
+    let replica_head = target_client
+        .head_object()
+        .bucket(target_bucket)
+        .key(key)
+        .sse_customer_algorithm("AES256")
+        .sse_customer_key(&customer_key)
+        .sse_customer_key_md5(&customer_key_md5)
+        .send()
+        .await?;
+    assert_eq!(
+        replica_head.replication_status().map(|status| status.as_str()),
+        Some("REPLICA"),
+        "the resynced copy must carry REPLICA status"
+    );
+    let replica = target_client
+        .get_object()
+        .bucket(target_bucket)
+        .key(key)
+        .sse_customer_algorithm("AES256")
+        .sse_customer_key(&customer_key)
+        .sse_customer_key_md5(&customer_key_md5)
+        .send()
+        .await?;
+    assert_eq!(replica.sse_customer_algorithm(), Some("AES256"));
+    assert_eq!(replica.body.collect().await?.into_bytes().as_ref(), body.as_slice());
+
+    // No plaintext leak: the replica stays unreadable without the key.
+    assert!(
+        target_client
+            .get_object()
+            .bucket(target_bucket)
+            .key(key)
+            .send()
+            .await
+            .is_err(),
+        "SSE-C replica must not be readable without the customer key"
+    );
+
+    Ok(())
+}
+
 /// backlog#1147 repl-17 / backlog#1783: SSE-S3 objects replicate by decrypting
 /// at the source and re-encrypting on the target with the target's own KMS.
 /// The property backlog#1291 pinned — never a silent plaintext replica — still

crates/ecstore/src/api/mod.rs

@@ -32,7 +32,7 @@ pub mod bucket {
     pub mod bucket_target_sys {
         pub use crate::bucket::bucket_target_sys::{
             AdvancedPutOptions, BucketTargetError, BucketTargetSys, PutObjectOptions, RemoveObjectOptions, S3ClientError,
-            TargetClient, append_version_id_query,
+            SsecPassthroughCapability, TargetClient, append_version_id_query,
         };
     }
 

crates/ecstore/src/bucket/bucket_target_sys.rs

@@ -299,9 +299,51 @@ struct TargetClientBuildProbe {
     release: Arc<tokio::sync::Semaphore>,
 }
 
+/// Whether a replication target preserves the SSE-C passthrough transport
+/// headers (`X-Rustfs-Replication-*`) end to end.
+///
+/// A target that silently drops those headers (MinIO, generic S3) stores the
+/// forwarded ciphertext without its decryption material — an unreadable
+/// replica that used to report COMPLETED. The replication worker audits the
+/// first passthrough PUT per target (HEAD-back for SSE-C evidence) and caches
+/// the verdict here; a fresh `Unsupported` fails SSE-C replication closed
+/// before any PUT is sent. Entries follow the `arn_remotes_map` lifecycle
+/// (rebuilding or removing a target resets its capability to `Unknown`) and
+/// additionally expire after [`SSEC_PASSTHROUGH_CAPABILITY_TTL`], after which
+/// the next attempt re-audits.
+#[derive(Debug, Clone, Copy, PartialEq, Eq, Default)]
+pub enum SsecPassthroughCapability {
+    #[default]
+    Unknown,
+    Supported,
+    Unsupported,
+}
+
+/// How long an audited SSE-C passthrough verdict stays authoritative.
+///
+/// Trade-off: without a TTL a verdict is sticky for the process lifetime —
+/// an `Unsupported` target that gets upgraded (or re-probed only via
+/// replication-check) would keep failing SSE-C replication forever, and the
+/// fail-open twin: a `Supported` verdict would outlive a backend swapped
+/// behind the same endpoint/ARN. With the TTL, a bad target costs at most
+/// one wasted PUT+HEAD audit per TTL window, and a changed backend is
+/// re-discovered within the same window.
+pub const SSEC_PASSTHROUGH_CAPABILITY_TTL: Duration = Duration::from_secs(10 * 60);
+
+/// A recorded SSE-C passthrough verdict plus when it was recorded, so reads
+/// can report staleness against [`SSEC_PASSTHROUGH_CAPABILITY_TTL`].
+#[derive(Debug, Clone, Copy)]
+struct SsecPassthroughRecord {
+    capability: SsecPassthroughCapability,
+    recorded_at: Instant,
+}
+
 #[derive(Debug, Default)]
 pub struct BucketTargetSys {
     pub arn_remotes_map: Arc<RwLock<HashMap<String, ArnTarget>>>,
+    /// SSE-C passthrough capability verdicts keyed by target ARN. See
+    /// [`SsecPassthroughCapability`]; reset alongside `arn_remotes_map`.
+    ssec_passthrough_map: Arc<RwLock<HashMap<String, SsecPassthroughRecord>>>,
     pub targets_map: Arc<RwLock<HashMap<String, Vec<BucketTarget>>>>,
     pub h_mutex: Arc<RwLock<HashMap<String, EpHealth>>>,
     target_h_mutex: Arc<RwLock<HashMap<String, EpHealth>>>,
@@ -322,6 +364,7 @@ impl BucketTargetSys {
     fn new() -> Self {
         Self {
             arn_remotes_map: Arc::new(RwLock::new(HashMap::new())),
+            ssec_passthrough_map: Arc::new(RwLock::new(HashMap::new())),
             targets_map: Arc::new(RwLock::new(HashMap::new())),
             h_mutex: Arc::new(RwLock::new(HashMap::new())),
             target_h_mutex: Arc::new(RwLock::new(HashMap::new())),
@@ -585,19 +628,59 @@ impl BucketTargetSys {
         let update_mutex = self.target_update_mutex(bucket).await;
         let _update_guard = update_mutex.lock().await;
 
-        // Lock order: targets_map, then arn_remotes_map, then target_h_mutex.
+        // Lock order: targets_map, then arn_remotes_map, then target_h_mutex,
+        // then ssec_passthrough_map (always last; also taken standalone by the
+        // capability accessors).
         let mut targets_map = self.targets_map.write().await;
         let mut arn_remotes_map = self.arn_remotes_map.write().await;
         let mut health_map = self.target_h_mutex.write().await;
 
         if let Some(targets) = targets_map.remove(bucket) {
+            let mut ssec_map = self.ssec_passthrough_map.write().await;
             for target in targets {
                 arn_remotes_map.remove(&target.arn);
                 health_map.remove(&target.arn);
+                ssec_map.remove(&target.arn);
             }
         }
     }
 
+    /// Cached SSE-C passthrough capability for a target ARN, plus whether the
+    /// verdict is older than [`SSEC_PASSTHROUGH_CAPABILITY_TTL`]. `(Unknown,
+    /// false)` when no verdict has been recorded since the target was built.
+    /// Staleness is computed here so the gate policy stays a pure function.
+    pub async fn ssec_passthrough_capability(&self, arn: &str) -> (SsecPassthroughCapability, bool) {
+        match self.ssec_passthrough_map.read().await.get(arn) {
+            Some(record) => (record.capability, record.recorded_at.elapsed() >= SSEC_PASSTHROUGH_CAPABILITY_TTL),
+            None => (SsecPassthroughCapability::Unknown, false),
+        }
+    }
+
+    /// Record an audited SSE-C passthrough verdict for a target ARN. Written by
+    /// the replication worker's HEAD-back audit and by the replication-check
+    /// SsecPassthrough probe phase.
+    pub async fn record_ssec_passthrough_capability(&self, arn: &str, capability: SsecPassthroughCapability) {
+        self.ssec_passthrough_map.write().await.insert(
+            arn.to_string(),
+            SsecPassthroughRecord {
+                capability,
+                recorded_at: Instant::now(),
+            },
+        );
+    }
+
+    /// Test hook: age an existing verdict so TTL expiry is observable without
+    /// waiting out the real window.
+    #[cfg(test)]
+    pub(crate) async fn backdate_ssec_passthrough_capability(&self, arn: &str, age: Duration) {
+        let backdated = Instant::now()
+            .checked_sub(age)
+            .expect("system uptime must exceed the backdate age");
+        if let Some(record) = self.ssec_passthrough_map.write().await.get_mut(arn) {
+            record.recorded_at = backdated;
+        }
+    }
+
     pub async fn set_target(
         &self,
         bucket: &str,
@@ -953,15 +1036,21 @@ impl BucketTargetSys {
             }
         }
 
-        // Lock order: targets_map, then arn_remotes_map, then target_h_mutex.
+        // Lock order: targets_map, then arn_remotes_map, then target_h_mutex,
+        // then ssec_passthrough_map (always last; also taken standalone by the
+        // capability accessors).
         let mut targets_map = self.targets_map.write().await;
         let mut arn_remotes_map = self.arn_remotes_map.write().await;
         let mut health_map = self.target_h_mutex.write().await;
         // Remove existing targets
         if let Some(existing_targets) = targets_map.remove(bucket) {
+            let mut ssec_map = self.ssec_passthrough_map.write().await;
             for target in existing_targets {
                 arn_remotes_map.remove(&target.arn);
                 health_map.remove(&target.arn);
+                // A rebuilt/edited target may point at a different service:
+                // the SSE-C passthrough verdict must be re-audited from Unknown.
+                ssec_map.remove(&target.arn);
                 self.update_bandwidth_limit(bucket, &target.arn, 0);
             }
         }
@@ -1455,7 +1544,7 @@ fn resolve_put_api_version_id(source_version_id: &str) -> Option<&str> {
 /// RustFS represents the null version internally as the nil UUID while the S3
 /// API addresses it as the literal "null" (same mapping as
 /// [`resolve_put_api_version_id`]); empty means "no version requested".
-fn resolve_read_api_version_id(version_id: Option<String>) -> Option<String> {
+pub(crate) fn resolve_read_api_version_id(version_id: Option<String>) -> Option<String> {
     let version_id = version_id?;
     let trimmed = version_id.trim();
     if trimmed.is_empty() {
@@ -2676,6 +2765,57 @@ mod tests {
         assert_eq!(health.last_online, Some(now));
     }
 
+    /// N2 TTL contract, both flip directions: a recorded verdict is fresh
+    /// until [`SSEC_PASSTHROUGH_CAPABILITY_TTL`], then reads as expired; a
+    /// re-audit that records the OPPOSITE verdict replaces it as fresh. The
+    /// worker gate maps expired verdicts to ProceedWithAudit (pinned in
+    /// `replication_target_boundary`), so together this proves an Unsupported
+    /// target recovers to Supported through the audit once its verdict ages
+    /// out — and a stale Supported one is re-proven rather than trusted.
+    #[tokio::test]
+    async fn ssec_passthrough_capability_ttl_expires_and_reaudit_flips_verdict() {
+        let sys = BucketTargetSys::default();
+        let arn = "arn:rustfs:replication:us-east-1:bucket:ssec-ttl";
+        let expired_age = SSEC_PASSTHROUGH_CAPABILITY_TTL + Duration::from_secs(1);
+
+        assert_eq!(
+            sys.ssec_passthrough_capability(arn).await,
+            (SsecPassthroughCapability::Unknown, false),
+            "an unrecorded target must read Unknown and never expired"
+        );
+
+        sys.record_ssec_passthrough_capability(arn, SsecPassthroughCapability::Unsupported)
+            .await;
+        assert_eq!(
+            sys.ssec_passthrough_capability(arn).await,
+            (SsecPassthroughCapability::Unsupported, false)
+        );
+
+        sys.backdate_ssec_passthrough_capability(arn, expired_age).await;
+        assert_eq!(
+            sys.ssec_passthrough_capability(arn).await,
+            (SsecPassthroughCapability::Unsupported, true),
+            "an aged-out Unsupported verdict must read expired so the gate re-audits"
+        );
+
+        // The re-audit against an upgraded target records Supported afresh.
+        sys.record_ssec_passthrough_capability(arn, SsecPassthroughCapability::Supported)
+            .await;
+        assert_eq!(
+            sys.ssec_passthrough_capability(arn).await,
+            (SsecPassthroughCapability::Supported, false),
+            "a fresh Supported verdict replaces the expired Unsupported one"
+        );
+
+        // And the fail-open twin: Supported also ages out.
+        sys.backdate_ssec_passthrough_capability(arn, expired_age).await;
+        assert_eq!(
+            sys.ssec_passthrough_capability(arn).await,
+            (SsecPassthroughCapability::Supported, true),
+            "an aged-out Supported verdict must read expired so the gate re-proves it"
+        );
+    }
+
     #[tokio::test]
     async fn list_targets_applies_health_stats_by_arn_and_preserves_endpoint_port() {
         let sys = BucketTargetSys::default();

crates/ecstore/src/bucket/replication/replication_resyncer.rs

@@ -49,10 +49,12 @@ use super::replication_storage_boundary::{
     ReplicationDeletedObject, ReplicationObjectIO, ReplicationStorage, StatObjectOptions, StorageObjectInfoOrErr, WalkOptions,
 };
 use super::replication_target_boundary::{
-    PutObjectOptions, PutObjectPartOptions, ReplicationTargetStore, TargetClient, replication_action_for_target_head,
+    ERR_REPLICATION_SSEC_PASSTHROUGH_UNSUPPORTED, PutObjectOptions, PutObjectPartOptions, ReplicationTargetStore,
+    SsecPassthroughCapability, SsecPassthroughGate, TargetClient, replication_action_for_target_head,
     replication_complete_multipart_options, replication_delete_marker_purge_remove_options, replication_delete_remove_options,
     replication_force_delete_remove_options, replication_object_is_ssec_encrypted, replication_put_object_header_size,
-    replication_put_object_options, replication_target_head_is_newer_null_version,
+    replication_put_object_options, replication_target_head_is_newer_null_version, resolve_read_api_version_id,
+    ssec_passthrough_evidence_present, ssec_passthrough_gate,
 };
 use super::replication_versioning_boundary::ReplicationVersioningStore;
 use super::runtime_boundary as runtime_sources;
@@ -279,6 +281,114 @@ async fn head_object_fallback(
     }
 }
 
+/// Resolve the N2 fail-closed gate for an SSE-C passthrough attempt against
+/// this target. Returns `Some(audit_required)` when replication may proceed;
+/// on a freshly-flagged header-dropping target it settles `rinfo` as FAILED
+/// (no PUT is ever sent — the object stays on the normal MRF retry channel
+/// and re-audits once the verdict's TTL expires or replication-check
+/// re-probes the target) and returns `None`.
+async fn resolve_ssec_passthrough_gate(
+    ssec: bool,
+    tgt_client: &TargetClient,
+    bucket: &str,
+    object: &str,
+    rinfo: &mut ReplicatedTargetInfo,
+) -> Option<bool> {
+    let (capability, expired) = ReplicationTargetStore::ssec_passthrough_capability(&tgt_client.arn).await;
+    match ssec_passthrough_gate(ssec, capability, expired) {
+        SsecPassthroughGate::Proceed => Some(false),
+        SsecPassthroughGate::ProceedWithAudit => Some(true),
+        SsecPassthroughGate::FailClosed => {
+            rinfo.replication_status = ReplicationStatusType::Failed;
+            rinfo.error = Some(ERR_REPLICATION_SSEC_PASSTHROUGH_UNSUPPORTED.to_string());
+            warn!(
+                event = EVENT_RESYNC_TARGET_OPERATION_FAILED,
+                component = LOG_COMPONENT_ECSTORE,
+                subsystem = LOG_SUBSYSTEM_REPLICATION_RESYNC,
+                bucket = %bucket,
+                object = %object,
+                arn = %tgt_client.arn,
+                operation = "ssec_passthrough_gate",
+                error = ERR_REPLICATION_SSEC_PASSTHROUGH_UNSUPPORTED,
+                "Replication target operation failed"
+            );
+            None
+        }
+    }
+}
+
+/// Judge SSE-C passthrough evidence on a HEAD of the replica and record the
+/// capability verdict for the target. Returns true when the SSE-C material
+/// provably survived; otherwise records `Unsupported` and settles `rinfo` as
+/// FAILED so the attempt never reports a silently unreadable COMPLETED.
+async fn settle_ssec_passthrough_evidence(
+    head: &HeadObjectOutput,
+    tgt_client: &TargetClient,
+    bucket: &str,
+    object: &str,
+    rinfo: &mut ReplicatedTargetInfo,
+) -> bool {
+    if ssec_passthrough_evidence_present(head) {
+        ReplicationTargetStore::record_ssec_passthrough_capability(&tgt_client.arn, SsecPassthroughCapability::Supported).await;
+        return true;
+    }
+    ReplicationTargetStore::record_ssec_passthrough_capability(&tgt_client.arn, SsecPassthroughCapability::Unsupported).await;
+    rinfo.replication_status = ReplicationStatusType::Failed;
+    rinfo.error = Some(ERR_REPLICATION_SSEC_PASSTHROUGH_UNSUPPORTED.to_string());
+    warn!(
+        event = EVENT_RESYNC_TARGET_OPERATION_FAILED,
+        component = LOG_COMPONENT_ECSTORE,
+        subsystem = LOG_SUBSYSTEM_REPLICATION_RESYNC,
+        bucket = %bucket,
+        object = %object,
+        arn = %tgt_client.arn,
+        endpoint = %tgt_client.endpoint,
+        operation = "ssec_passthrough_audit",
+        error = ERR_REPLICATION_SSEC_PASSTHROUGH_UNSUPPORTED,
+        "Replication target operation failed"
+    );
+    false
+}
+
+/// Post-PUT HEAD-back audit for an SSE-C passthrough replica, over the worker
+/// HEAD channel (replication-check exemption plus the `source-proxy-request:
+/// false` suppression header, so the target answers locally without a
+/// customer key). A HEAD transport failure leaves the capability `Unknown`
+/// but still fails this attempt: an unverifiable SSE-C replica must not
+/// report COMPLETED.
+async fn audit_ssec_passthrough_replica(
+    tgt_client: &Arc<TargetClient>,
+    bucket: &str,
+    object: &str,
+    version_id: Option<String>,
+    rinfo: &mut ReplicatedTargetInfo,
+) -> bool {
+    // Address the replica the way the PUT named it: a nil source version id
+    // (versioning-suspended / null-version objects) maps to the "null"
+    // version, so the audit HEAD does not 4xx-loop on those objects.
+    let version_id = resolve_read_api_version_id(version_id);
+    match head_object_for_worker(tgt_client.as_ref(), &tgt_client.bucket, object, version_id).await {
+        Ok(head) => settle_ssec_passthrough_evidence(&head, tgt_client, bucket, object, rinfo).await,
+        Err(e) => {
+            rinfo.replication_status = ReplicationStatusType::Failed;
+            rinfo.error = Some(format!("SSE-C passthrough audit HEAD failed: {e}"));
+            warn!(
+                event = EVENT_RESYNC_TARGET_OPERATION_FAILED,
+                component = LOG_COMPONENT_ECSTORE,
+                subsystem = LOG_SUBSYSTEM_REPLICATION_RESYNC,
+                bucket = %bucket,
+                object = %object,
+                arn = %tgt_client.arn,
+                operation = "ssec_passthrough_audit_head",
+                error = %e,
+                "Replication target operation failed"
+            );
+            mark_replication_target_offline_if_needed(tgt_client, &e).await;
+            false
+        }
+    }
+}
+
 static RESYNC_WORKER_COUNT: usize = 10;
 
 fn resync_status_duration(
@@ -2872,6 +2982,21 @@ impl ReplicateObjectInfoExt for ReplicateObjectInfo {
             return rinfo;
         }
 
+        // N2 fail-closed: never PUT SSE-C ciphertext at a target known to drop
+        // the passthrough transport headers, and never trust a convergence HEAD
+        // against such a target — a previous broken replica matches by ETag.
+        let Some(ssec_audit_required) = resolve_ssec_passthrough_gate(self.ssec, &tgt_client, &bucket, &object, &mut rinfo).await
+        else {
+            send_local_event(EventArgs {
+                event_name: EventName::ObjectReplicationNotTracked.to_string(),
+                bucket_name: bucket.clone(),
+                object: self.to_object_info(),
+                user_agent: "Internal: [Replication]".to_string(),
+                ..Default::default()
+            });
+            return rinfo;
+        };
+
         let versioned = ReplicationVersioningStore::prefix_enabled(&bucket, &object).await;
         let version_suspended = ReplicationVersioningStore::prefix_suspended(&bucket, &object).await;
 
@@ -2975,6 +3100,14 @@ impl ReplicateObjectInfoExt for ReplicateObjectInfo {
             Ok(oi) => {
                 replication_action = replication_action_for_target_head(&object_info, &oi, self.op_type);
                 if replication_action == ReplicationAction::None {
+                    // An SSE-C replica only counts as converged when the same
+                    // HEAD proves its decryption material survived; a broken
+                    // ciphertext copy from an earlier attempt matches by ETag.
+                    if ssec_audit_required
+                        && !settle_ssec_passthrough_evidence(&oi, &tgt_client, &bucket, &object, &mut rinfo).await
+                    {
+                        return rinfo;
+                    }
                     rinfo.replication_status = ReplicationStatusType::Completed;
                     rinfo.replication_resynced = true;
                     rinfo.replication_action = ReplicationAction::None;
@@ -2989,6 +3122,11 @@ impl ReplicateObjectInfoExt for ReplicateObjectInfo {
                     // Version-ID format mismatch: retry without versionId and compare ETags.
                     match head_object_fallback(&tgt_client, &object).await {
                         Ok(Some(oi)) if replication_etags_match(object_info.etag.as_deref(), oi.e_tag.as_deref()) => {
+                            if ssec_audit_required
+                                && !settle_ssec_passthrough_evidence(&oi, &tgt_client, &bucket, &object, &mut rinfo).await
+                            {
+                                return rinfo;
+                            }
                             rinfo.replication_status = ReplicationStatusType::Completed;
                             rinfo.replication_resynced = true;
                             rinfo.replication_action = ReplicationAction::None;
@@ -3113,6 +3251,15 @@ impl ReplicateObjectInfoExt for ReplicateObjectInfo {
             return rinfo;
         }
 
+        // First SSE-C passthrough PUT against this target: verify the replica
+        // kept its decryption material before reporting COMPLETED.
+        if ssec_audit_required
+            && !audit_ssec_passthrough_replica(&tgt_client, &bucket, &object, self.version_id.map(|v| v.to_string()), &mut rinfo)
+                .await
+        {
+            return rinfo;
+        }
+
         rinfo.replication_status = ReplicationStatusType::Completed;
 
         rinfo
@@ -3135,6 +3282,21 @@ impl ReplicateObjectInfoExt for ReplicateObjectInfo {
             return rinfo;
         }
 
+        // N2 fail-closed: see the gate in `replicate_object` — the same policy
+        // applies to the metadata/existing-object transport.
+        let Some(ssec_audit_required) = resolve_ssec_passthrough_gate(self.ssec, &tgt_client, &bucket, &object, &mut rinfo).await
+        else {
+            send_local_event(EventArgs {
+                event_name: EventName::ObjectReplicationNotTracked.to_string(),
+                bucket_name: bucket.clone(),
+                object: self.to_object_info(),
+                user_agent: "Internal: [Replication]".to_string(),
+                ..Default::default()
+            });
+            rinfo.duration = (OffsetDateTime::now_utc() - start_time).unsigned_abs();
+            return rinfo;
+        };
+
         let versioned = ReplicationVersioningStore::prefix_enabled(&bucket, &object).await;
         let version_suspended = ReplicationVersioningStore::prefix_suspended(&bucket, &object).await;
 
@@ -3173,8 +3335,19 @@ impl ReplicateObjectInfoExt for ReplicateObjectInfo {
 
         let _sopts = replicate_all_stat_options(&object_info, &bucket, &tgt_client);
 
-        let Some((replication_action, object_info)) =
-            resolve_replicate_all_action(self, &tgt_client, &bucket, &object, object_info, start_time, &mut rinfo).await
+        let Some((replication_action, object_info)) = resolve_replicate_all_action(
+            ReplicateAllActionContext {
+                roi: self,
+                tgt_client: &tgt_client,
+                bucket: &bucket,
+                object: &object,
+                start_time,
+                ssec_audit_required,
+            },
+            object_info,
+            &mut rinfo,
+        )
+        .await
         else {
             return rinfo;
         };
@@ -3229,6 +3402,16 @@ impl ReplicateObjectInfoExt for ReplicateObjectInfo {
             return rinfo;
         }
 
+        // First SSE-C passthrough PUT against this target: verify the replica
+        // kept its decryption material before reporting COMPLETED.
+        if ssec_audit_required
+            && !audit_ssec_passthrough_replica(&tgt_client, &bucket, &object, self.version_id.map(|v| v.to_string()), &mut rinfo)
+                .await
+        {
+            rinfo.duration = (OffsetDateTime::now_utc() - start_time).unsigned_abs();
+            return rinfo;
+        }
+
         rinfo
     }
 
@@ -3441,25 +3624,49 @@ fn apply_replication_resync_timestamp(rinfo: &mut ReplicatedTargetInfo, reset_id
     rinfo.replication_resynced = true;
 }
 
+/// Borrowed inputs for [`resolve_replicate_all_action`].
+struct ReplicateAllActionContext<'a> {
+    roi: &'a ReplicateObjectInfo,
+    tgt_client: &'a Arc<TargetClient>,
+    bucket: &'a str,
+    object: &'a str,
+    start_time: OffsetDateTime,
+    /// N2: the target's SSE-C passthrough capability is still `Unknown`, so a
+    /// converged-looking replica must additionally prove its SSE-C material
+    /// survived before the comparison may settle COMPLETED.
+    ssec_audit_required: bool,
+}
+
 /// Compare the source object against the target via HEAD and decide which
 /// replication action is still required. Returns `None` after fully settling
 /// `rinfo` when replication must stop here — either because the target already
 /// matches or because the comparison failed.
 async fn resolve_replicate_all_action(
-    roi: &ReplicateObjectInfo,
-    tgt_client: &Arc<TargetClient>,
-    bucket: &str,
-    object: &str,
+    ctx: ReplicateAllActionContext<'_>,
     object_info: ObjectInfo,
-    start_time: OffsetDateTime,
     rinfo: &mut ReplicatedTargetInfo,
 ) -> Option<(ReplicationAction, ObjectInfo)> {
+    let ReplicateAllActionContext {
+        roi,
+        tgt_client,
+        bucket,
+        object,
+        start_time,
+        ssec_audit_required,
+    } = ctx;
     let replication_action;
     match head_object_for_worker(tgt_client.as_ref(), &tgt_client.bucket, object, roi.version_id.map(|v| v.to_string())).await {
         Ok(oi) => {
             replication_action = replication_action_for_target_head(&object_info, &oi, roi.op_type);
             rinfo.replication_status = ReplicationStatusType::Completed;
             if replication_action == ReplicationAction::None {
+                // An SSE-C replica only counts as converged when the same HEAD
+                // proves its decryption material survived; a broken ciphertext
+                // copy from an earlier attempt matches by ETag.
+                if ssec_audit_required && !settle_ssec_passthrough_evidence(&oi, tgt_client, bucket, object, rinfo).await {
+                    rinfo.duration = (OffsetDateTime::now_utc() - start_time).unsigned_abs();
+                    return None;
+                }
                 if roi.op_type == ReplicationType::ExistingObject
                     && replication_target_head_is_newer_null_version(&object_info, &oi)
                 {
@@ -3509,6 +3716,12 @@ async fn resolve_replicate_all_action(
                 match head_object_fallback(tgt_client, object).await {
                     Ok(Some(oi)) => {
                         replication_action = if replication_etags_match(object_info.etag.as_deref(), oi.e_tag.as_deref()) {
+                            if ssec_audit_required
+                                && !settle_ssec_passthrough_evidence(&oi, tgt_client, bucket, object, rinfo).await
+                            {
+                                rinfo.duration = (OffsetDateTime::now_utc() - start_time).unsigned_abs();
+                                return None;
+                            }
                             ReplicationAction::None
                         } else {
                             ReplicationAction::All

crates/ecstore/src/bucket/replication/replication_target_boundary.rs

@@ -36,7 +36,8 @@ use time::OffsetDateTime;
 use time::format_description::well_known::Rfc3339;
 
 pub(crate) use crate::bucket::bucket_target_sys::{
-    AdvancedPutOptions, PutObjectOptions, PutObjectPartOptions, RemoveObjectOptions, TargetClient,
+    AdvancedPutOptions, PutObjectOptions, PutObjectPartOptions, RemoveObjectOptions, SsecPassthroughCapability, TargetClient,
+    resolve_read_api_version_id,
 };
 #[cfg(test)]
 pub(crate) use crate::bucket::target::BucketTarget;
@@ -65,6 +66,8 @@ static STANDARD_HEADERS: &[&str] = &[
 ];
 
 const ERR_REPLICATION_ENCRYPTION_METADATA_UNSUPPORTED: &str = "replication source contains unsupported encryption metadata";
+pub(crate) const ERR_REPLICATION_SSEC_PASSTHROUGH_UNSUPPORTED: &str = "replication target does not support SSE-C passthrough: the replica would lose its decryption material \
+     (run ?replication-check to re-probe)";
 
 #[derive(Debug, Clone, Copy, PartialEq, Eq)]
 enum ReplicationSourceEncryption {
@@ -146,6 +149,54 @@ pub(crate) fn replication_object_is_ssec_encrypted(user_defined: &HashMap<String
     rustfs_replication::is_ssec_encrypted(user_defined)
 }
 
+/// Fail-closed decision for an SSE-C passthrough replication attempt, derived
+/// from the target's cached [`SsecPassthroughCapability`]. Pure so the policy
+/// can migrate with the worker (M2) without dragging the cache along; the
+/// caller computes `expired` from the cache record's age (see
+/// `SSEC_PASSTHROUGH_CAPABILITY_TTL`).
+#[derive(Debug, Clone, Copy, PartialEq, Eq)]
+pub(crate) enum SsecPassthroughGate {
+    /// Not an SSE-C object, or the target has a fresh proof that it preserves
+    /// the passthrough transport headers: replicate without a HEAD-back audit.
+    Proceed,
+    /// No usable verdict — first SSE-C attempt since the target was (re)built,
+    /// or the recorded verdict (in either direction) aged out: PUT, then HEAD
+    /// the replica back and require SSE-C evidence before reporting COMPLETED.
+    ProceedWithAudit,
+    /// The target was recently proven to drop the passthrough headers: do not
+    /// send the PUT, report FAILED (the object stays on the normal MRF retry
+    /// channel and re-audits once the verdict expires).
+    FailClosed,
+}
+
+pub(crate) fn ssec_passthrough_gate(ssec: bool, capability: SsecPassthroughCapability, expired: bool) -> SsecPassthroughGate {
+    if !ssec {
+        return SsecPassthroughGate::Proceed;
+    }
+    // An expired verdict — Supported or Unsupported — must be re-earned: a
+    // stale Unsupported would otherwise stick forever after a target upgrade,
+    // and a stale Supported would fail open after a backend swap behind the
+    // same endpoint.
+    if expired {
+        return SsecPassthroughGate::ProceedWithAudit;
+    }
+    match capability {
+        SsecPassthroughCapability::Supported => SsecPassthroughGate::Proceed,
+        SsecPassthroughCapability::Unknown => SsecPassthroughGate::ProceedWithAudit,
+        SsecPassthroughCapability::Unsupported => SsecPassthroughGate::FailClosed,
+    }
+}
+
+/// True when a replication-check HEAD of the replica proves the SSE-C
+/// material survived passthrough: a RustFS target restores the transport
+/// headers into the stored SSE-C keys and its HEAD echoes
+/// `x-amz-server-side-encryption-customer-algorithm` (the replication-check
+/// exemption skips key validation but not the metadata echo). A target that
+/// dropped the headers stored a plain object and echoes nothing.
+pub(crate) fn ssec_passthrough_evidence_present(head: &HeadObjectOutput) -> bool {
+    head.sse_customer_algorithm.as_deref().is_some_and(|algo| !algo.is_empty())
+}
+
 pub(crate) struct ReplicationTargetStore;
 
 impl ReplicationTargetStore {
@@ -165,6 +216,17 @@ impl ReplicationTargetStore {
         BucketTargetSys::get().mark_target_offline(target_client).await
     }
 
+    /// Returns the cached verdict and whether it has outlived its TTL.
+    pub(crate) async fn ssec_passthrough_capability(arn: &str) -> (SsecPassthroughCapability, bool) {
+        BucketTargetSys::get().ssec_passthrough_capability(arn).await
+    }
+
+    pub(crate) async fn record_ssec_passthrough_capability(arn: &str, capability: SsecPassthroughCapability) {
+        BucketTargetSys::get()
+            .record_ssec_passthrough_capability(arn, capability)
+            .await
+    }
+
     #[cfg(test)]
     pub(crate) async fn register_test_target(target_client: &Arc<TargetClient>) {
         BucketTargetSys::get().arn_remotes_map.write().await.insert(
@@ -898,6 +960,71 @@ mod tests {
         }
     }
 
+    /// N2 fail-closed policy: SSE-C replication may only proceed silently
+    /// against a target with a FRESH proof that it preserves the passthrough
+    /// transport headers. Unknown targets must be audited; freshly-flagged
+    /// dropping targets must never receive the PUT; an expired verdict in
+    /// EITHER direction must be re-earned through the audit — a sticky
+    /// Unsupported would outlive a target upgrade, and a sticky Supported
+    /// would fail open after a backend swap behind the same endpoint.
+    #[test]
+    fn ssec_passthrough_gate_is_fail_closed_and_ttl_bounded() {
+        for capability in [
+            SsecPassthroughCapability::Unknown,
+            SsecPassthroughCapability::Supported,
+            SsecPassthroughCapability::Unsupported,
+        ] {
+            for expired in [false, true] {
+                assert_eq!(
+                    ssec_passthrough_gate(false, capability, expired),
+                    SsecPassthroughGate::Proceed,
+                    "non-SSE-C objects must never be gated on the passthrough capability"
+                );
+            }
+        }
+        assert_eq!(
+            ssec_passthrough_gate(true, SsecPassthroughCapability::Supported, false),
+            SsecPassthroughGate::Proceed
+        );
+        assert_eq!(
+            ssec_passthrough_gate(true, SsecPassthroughCapability::Unknown, false),
+            SsecPassthroughGate::ProceedWithAudit
+        );
+        assert_eq!(
+            ssec_passthrough_gate(true, SsecPassthroughCapability::Unsupported, false),
+            SsecPassthroughGate::FailClosed
+        );
+        // Expiry flips both directions back to the audit.
+        assert_eq!(
+            ssec_passthrough_gate(true, SsecPassthroughCapability::Unsupported, true),
+            SsecPassthroughGate::ProceedWithAudit,
+            "an expired Unsupported verdict must allow a re-audit (upgraded target recovers without operator action)"
+        );
+        assert_eq!(
+            ssec_passthrough_gate(true, SsecPassthroughCapability::Supported, true),
+            SsecPassthroughGate::ProceedWithAudit,
+            "an expired Supported verdict must be re-proven (backend swap behind the same endpoint must not fail open)"
+        );
+    }
+
+    #[test]
+    fn ssec_passthrough_evidence_requires_customer_algorithm_echo() {
+        let with_evidence = HeadObjectOutput::builder().sse_customer_algorithm("AES256").build();
+        assert!(ssec_passthrough_evidence_present(&with_evidence));
+
+        let empty_algorithm = HeadObjectOutput::builder().sse_customer_algorithm("").build();
+        assert!(
+            !ssec_passthrough_evidence_present(&empty_algorithm),
+            "an empty echo is not evidence of preserved SSE-C material"
+        );
+
+        let without_evidence = HeadObjectOutput::builder().e_tag("\"abc\"").content_length(8).build();
+        assert!(
+            !ssec_passthrough_evidence_present(&without_evidence),
+            "a plain HEAD response must classify the target as having dropped the material"
+        );
+    }
+
     #[test]
     fn replication_put_options_adds_ssec_checksum_metadata() {
         let metadata = HashMap::from([(SSEC_ALGORITHM_HEADER.to_string(), "AES256".to_string())]);